The present invention relates to an apparatus and method for controlling the irradiation of an electron beam at a fixed position in an electron beam tester system in which an electron beam is irradiated onto the surface of a sample to be measured such as, e.g., a large scale integrated circuit (LSI) and thereby measuring the state (potential) of the surface and, more particularly, to an apparatus and method for correcting the deviation of the irradiating position due to the drift of the electron beam which is caused during measurement.
In a tester system using an electron beam such as a scanning electron microscope apparatus (SEM), or stroboscopic scanning electron microscope apparatus, or the like, an electron beam is irradiated onto the surface of a sample to be measured and the resultant secondary electron or reflected electron which will be obtained in accordance with the state (material, potential, and the like) of the surface of the sample is detected, thereby displaying the state of the surface of the sample (in the case where the sample is an LSI, circuit pattern, potential distribution, and the like) on an image display device and observing it.
Hitherto, in the case of irradiating an electron beam onto a specific pattern formed on the surface of the LSI using such an electron beam tester system as mentioned above and thereby measuring a change in voltage of the specific pattern, the specific pattern is searched from among the patterns of the LSI and the electron beam is irradiated to the central position of this pattern. Due to this, the risk that the irradiating position of the electron beam deviates from the specific pattern due to drift of the electron beam during measurement of the sample to be measured is desired to be reduced as much as possible. However, when the electron beam drifts and deviates from the pattern, the irradiating position of the electron beam is reset and the measurement is again performed. The drift of the electron beam is mainly caused due to the following factors (1) to (4).
(1) The primary electron beam is bent by the electrical charge caused by the dust and the like deposited on the inner periphery of the electron-optical-column of the electron beam tester system.
(2) The primary electron beam is bent by the electrical charge accumulated in the surface of the sample as the electron beam is applied onto the sample.
(3) The sample to be measured expands due to the heat which is generated while the sample being measured is electrically made operative, so that the positional relations among the sample to be measured, package, performance board, and the stage (XY stage) on which the sample is put upon deviate.
(4) The beam alignment of the electron gun deviates due to a change in temperature of the electron gun during the measurement.
In the case where the dimension of the pattern onto which the electron beam will be irradiated is sufficiently large (e.g., 4 to 6 .mu.m), even if a slight drift of the electron beam occurs, the electron beam will hardly deviate from the pattern, so that no problem will occur. However, when the pattern dimension is 2 .mu.m or less, even if the electron beam is first set so as to irradiate the central position of the pattern as well, the possibility will increase that the irradiating position will be deviated from the pattern due to a drift of the electron beam during the measurement so much that the measurement cannot be performed. Therefore, the reliability of the electron beam tester system deteriorates. In addition, the necessity of remeasurement causes a reduction in the usability of the expensive electron beam tester system and a decrease in the measurement efficiency. Further, in this remeasurement, the original (measured) specific pattern needs to be determined once more. If this pattern is characteristic, it can be relatively easily searched by observing the patterns which are displayed on the image display device. However, in the case of a pattern image which is not characteristic (e.g., fine repetitive pattern), it is difficult to search for it. This also causes the reduction of the use efficiency of the electron beam tester system.